Nonaqueous electrolyte secondary battery has relatively high energy density. Using this advantage, in recent years, they have been widespread as power source for small consumer equipment such as mobile devices typified by mobile phone. In addition to applications to small consumer equipment, nonaqueous electrolyte secondary battery are expected to be developed also for medium-size and large-size industrial applications for electricity storage, electric vehicle, hybrid vehicle, or the like.
A nonaqueous electrolyte secondary battery generally includes a positive electrode containing a positive active material, a negative electrode containing a negative active material, a separator, and a nonaqueous electrolyte containing a nonaqueous solvent and an electrolyte salt.
As a positive active material forming a nonaqueous electrolyte secondary battery, a lithium-containing transition metal oxide is widely known, while as a negative active material, a carbon material such as graphite is widely known. As a nonaqueous electrolyte, one obtained by dissolving an electrolyte salt such as lithium hexafluorophosphate (LiPF6) in a nonaqueous solvent containing ethylene carbonate as a main constituent is widely known.
Nowadays, a large number of materials are known as positive active materials for nonaqueous electrolyte secondary batteries such as lithium ion secondary battery. Examples of the most commonly known positive active materials include lithium-containing transition metal oxides having an operating potential of about 4 V, whose basic structure is lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4) with a spinel-type structure, or the like. In particular, LiCoO2 is excellent in terms of charge and discharge performance and energy density, and thus has been widely adopted as a positive active material for small-capacity lithium ion secondary batteries having a battery capacity of up to 2 Ah.
However, in consideration of future development for applications to medium-size or large-size batteries, particularly for industrial applications that are expected to be in great market, because a battery for industrial applications is possibly used in a high-temperature environment where a small consumer battery would not be used, the safety of a battery is of extreme importance. In addition, in such a high-temperature environment, not only a lithium ion secondary battery but also a nickel-hydrogen battery, a nickel-cadmium battery, or a lead battery has an extremely short life. Therefore, in the present situation, there is no battery that satisfies users' demand. Meanwhile, a capacitor that has a relatively long life even in such a high-temperature environment has low energy density, and does not satisfy users' demand. Accordingly, there is a need for a battery that has high safety and a long life even in a high-temperature environment and also has high energy density.
As a countermeasure, lithium iron phosphate (LiFePO4) having an olivine-type structure has been proposed as a positive active material having excellent safety. In LiFePO4, oxygen and phosphorus have a covalent bond. Therefore, no oxygen gas or the like is generated even in a high-temperature environment, and safety is thus high.
However, in LiFePO4, the insertion/extraction of lithium takes place at a potential of about 3.4 V relative to the lithium metal potential. Therefore, there is a problem in that its energy density is low as compared with conventional lithium-containing transition metal oxides.
Then, as a positive active material having relatively high energy density and excellent safety, attention has recently been focused on a lithium-containing phosphate compound having a NASICON-type structure, in which the insertion/extraction of lithium takes place at about 4 V relative to the lithium metal potential. A typical example of such a lithium-containing phosphate compound includes lithium vanadium phosphate (Li3V2(PO4)3). Li3V2(PO4)3 has a high lithium content per formula weight, and when all Li is extracted, the theoretical capacity is 197 mAh/g. Therefore, it is expected to serve as a positive active material having both high safety and high energy density.
Patent Document 1 discloses the invention of “a lithium secondary battery including a first electrode containing an electrode active material represented by the nominal general formula Li3-xM′yM″2-y(PO4)3 (wherein M′ and M″ are the same or different, at least one of M′ and M″ has more than one oxidation state, and 0≦y≦2), a second counter electrode containing an intercalation active material, and an electrolyte, wherein x=0 under a first condition and 0≦x≦3 under a second condition, M′ and M″ are each a metal or a semimetal, and at least one of M′ and M″ has a higher oxidation state than the oxidation state under the first condition” (claim 1) and “the lithium secondary battery according to claim 1, wherein M′ and M″ are each independently selected from the group consisting of Ti (titanium), V (vanadium), Cr (chromium), Mn (manganese), Fe (iron), Co (cobalt), Mo (molybdenum), and Cu (copper)” (claim 5).
Patent Document 1 states the following: “The present invention provides a lithium metal phosphate compound containing an oxidizable metal. Such a metal can have more than one oxidation state. The metal in the lithium metal phosphate compound exists in an oxidation state lower than the highest oxidation state. Therefore, the metal can be oxidized, providing the ability of extracting one or more Li+ ions” (page 13, lines 18 to 22); as a result, it is possible that “the object, feature, and advantage of the present invention include an improved lithium-based electrochemical cell or battery that has improved charge-discharge characteristics and high discharge capacity and maintains the complete state during cycle operation” (page 6, lines 42 to 44). As the above active material, Li3V2(PO4)3 is shown in the Examples of Patent Document 1, and also Li3FeV(PO4)3 and Li3AlTm(PO4)3 are mentioned as examples in the Best Mode for Carrying Out the Invention.
However, Patent Document 1 nowhere states or suggests that cycle performance is significantly improved when, in the general formula Li3-xM′yM″2-y(PO4)3, Fe and V are selected as M′ and M″, and the value of y is within a range of 0.04 to 0.4.
Patent Document 2 discloses the invention of “a lithium ion battery including a first electrode having an active material whose first condition is the nominal general formula Li3-xM′yM″2-y(PO4)3 wherein x=0 and 0≦y≦2 and second condition is the nominal general formula Li3-xM′yM″2-y(PO4)3 wherein 0<x≦3, wherein M″ is a transition metal, and M′ is a non-transition metal element selected from the group consisting of metals and metalloids; a second electrode that is a counter electrode opposed to the first electrode; and an electrolyte between the electrodes” (claim 1).
Patent Document 2 states that the use of the above active material is advantageous in that “the object, feature, and advantage of the present invention include an improved lithium-based electrochemical cell or battery that has improved charge-discharge characteristics and high discharge capacity and also maintains its completeness during charge-discharge” (page 11, lines 1 to 3). In the Examples of Patent Document 2, it is stated that various Li3-xM′M″ (PO4)3 and Li3-xMIIMIV(PO4)3 such as Li3V2(PO4)3 and Li3AlV(PO4)3 show excellent charge-discharge reversibility and capacity. However, Patent Document 2 nowhere describes or suggests that V and Fe are selected as M′ and M.″
Patent Document 3 discloses the invention of a lithium-vanadium phosphate composite compound substituted a part of vanadium, and states the following: “In the lithium-vanadium phosphate composite compound of the present invention, vanadium is partially substituted with Zr, Ti, and/or Al; as a result, the high-temperature phase, which is usually stable at high temperatures, is stabilized even at room temperature. Accordingly, the high-temperature phase stabilized at room temperature leads to significantly improved positive electrode characteristics. That is, according to the invention of the present application, the high-temperature phase having high ion conductivity and ion diffusibility is stabilized at room temperature, whereby the low charge-discharge capacity, a disadvantage of Li3V2(PO4)3 and Li3Fe2(PO4)3, is improved” (paragraph 0009). In the Examples of Patent Document 3, a compound obtained by partially substituting vanadium of a lithium-vanadium phosphate composite compound with Al, Ti, or Zr in an amount within a range of 5 to 20 mol % is shown, and it is shown that “by substituting a predetermined amount of vanadium with at least one kind of divalent or higher-valent cation selected from aluminum, titanium, and zirconium, the ion-conducting phase that is stable at high temperatures is stabilized also at room temperature, whereby ion conductivity is improved, ion diffusibility is increased, and charge-discharge capacity is improved” (paragraph 0029).
Patent Document 4 relates to a 3V battery for memory backup and discloses the invention of “a lithium ion secondary battery including a positive electrode containing as an active material a NASICON-type compound represented by the chemical formula LinM2(XO4)3 (wherein n, M, and X are each as follows: 0≦n≦3, M=at least one metal element selected from Al, Ti, Ni, V, Nb, and Mn, and X=P, S, Mo, W, and As), a negative electrode containing a carbonaceous material capable of electrochemically inserting/extracting lithium, and a nonaqueous electrolyte solution” (claim 1). Patent Document 4 states the following: “Because of the inclusion of the positive electrode containing as an active material a NASICON-type compound represented by the chemical formula LinM2(XO4)3 (wherein n, M, and X are each as follows: 0≦n≦3, M=at least one metal element selected from Fe, Ti, Ni, V, Nb, and Mn, and X=P, S, Mo, W, and As), a negative electrode containing a lithium-containing carbonaceous material capable of electrochemically inserting and extracting lithium, and a nonaqueous electrolyte solution, it is possible to provide a long-life lithium ion secondary battery that shows a stable operating voltage, shows an excellent capacity retention ratio, and has excellent cycle performance. In addition, particularly in the case of using a V2(SO4)3 NASICON-type compound as a positive active material, a long-life 3V battery suitable for backup was achieved” (paragraph 0033). In the Example of Patent Document 4, V2(SO4)3 and LiTi2(PO4)3 are shown as compounds that satisfy the above chemical formula.
Patent Document 5 discloses the invention that relates to “an active material for lithium secondary batteries, which is represented by the general formula LiaMb(PO4)1-x(BO3)x (wherein M is one or more kinds of transition metal elements selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, and Ni, 0<a, 0<b, 0.5<a+b≦2, 0<x<1, and a and b are selected such that the general formula maintains electrical neutrality)” (claim 1). Patent Document 5 describes the partial substitution of PO4 with BO3 for the purpose of “providing a polyanionic active material having excellent high rate discharge performance and a lithium secondary battery using the same” (paragraph 0014). In the Example of Patent Document 5, it is shown that in Li3V2(PO4)3-x(BO3)x having PO4 partially substituted with BO3, “when x is within a range of 1/64 to ¼, surprisingly, the high rate discharge characteristic value is found improved over Li3V2(PO4)3 (x=0)” (paragraph 0061).
However, the lithium vanadium phosphate compounds and derivatives thereof shown in these prior art documents have problems in that they are not necessarily excellent in terms of the performance of battery capacity retention after repeated charge-discharge cycle of a battery. That is, there is a problem in that the cycle performance is not necessarily excellent.